Continuously variable transmissions (hereinafter referred to as a CVT) are known, which substantially comprise an input shaft; and a drive pulley connectable to the input shaft and comprising two half-pulleys defining a V groove of variable size to vary the wind diameter of a V belt. The pulley is located between a friction disk, connected rigidly to the input shaft, and a disk push plate angularly fixed but axially slidable with respect to the input shaft.
In automatic mechanical solutions, respective friction seals are interposed between each half-pulley and the friction disk and disk push plate, and a centrifugal control device cooperates with the disk push plate to move it axially towards the pulley by an amount varying as a function of the speed of the shaft.
More specifically, in one known solution, the control device comprises a hub fixed rigidly to the shaft; and a number of centrifugal weights carried by the hub and which exert centrifugal axial thrust on the disk push plate to first connect the pulley to the input shaft via the friction seals, and then gradually reduce the distance between the half-pulleys as the angular speed of the input shaft increases.
Known CVT transmissions of the type briefly described above are widely used on low-power motorcycles, in particular scooters, but have drawbacks when used, as frequently is the case, in higher-power applications, such as so-called minicars.
More specifically, response of the control device to variations in shaft speed, i.e. to the accelerator, is irregular and results in “tugging” of the transmission, which is clearly noticeable when accelerating and decelerating, and particularly at low engine speeds, when starting up and parking.
Another drawback of known transmissions is relatively severe wear of the belt, caused by the belt slipping with respect to the half-pulleys at start-up, when very little axial thrust is exerted on the half-pulleys by the control device.
To eliminate these drawbacks, CVT's for minicars have been devised in which the centrifugal control device comprises a centrifugal actuating device, which comes into play above a first input shaft angular speed threshold value to connect the drive pulley angularly to the flywheel by a friction clutch; and a speed regulator, which is activated above a second input shaft angular speed threshold value to vary the size of the drive pulley groove and, therefore, the work diameter of the belt.